power drop under load mercruiser 3.0

[Sashap]

Ok here goes. I have been having issues on my mercruiser 3.0 1992. Serial D706324
When idling or anything up to about 1/3 throttle it seems great, but after that there is a noticeable drop in power and the engine seems to stumble/bog down and actually drops speed. Can't get it past say 10mph. Not prop related for sure as that would not cause even lower engine rpm above 1/3 throttle, but would stay flat.

Symptoms/specs/things done:

• Was not run for over 7 years - bought it this way.
• no real idea how many hours on motor as hour meter shows 70 hrs but I don't think it was a stock item as other photos boats of same year don't have it installed where mine is installed (below instrument panel at left of steering wheel)
• Carb was all corroded inside and I cleaned it/restored it.
• starts a bit hard ( I know the starting procedure. turn over 5 secs to load bowl, pump 3 times throttle, then start). It takes about 10-20 secs to start.
• idles good.
• low power in gear good.
• rpm in idle good.
• tach not working (new one on order) so I don't know what it could indicate.
• sometimes backfires/spits through carb on acceleration.
• starts harder when warm after sitting for a few minutes
• timing is currently 0 degrees.
• timing advances on increased rpm.
• plugs black, sooty indicating rich fuel.
• Carb mechanicals all smooth and clean
• all carb passages tested with air and by pumping fuel through with hand pump.
• carb power piston plunger was seized so I cleaned/polished it but unsure if it is operational at high vacuum/low rpm - this does not however explain lack of high speed as plunger would be at maximum extension and pushing on high speed valve at bottom of bowl anyway.
• accelerator pump is good and pumps nicely on throttle.
• flame arrestor clean
• spark on all cylinders checked using timing light
• cleaned distributor cap and rotor
• fuel pump and filters all good and clean
• compression 130 on all cylinders
• emptied fuel, added bottle of seafoam and new fuel
• cleaned carb
• new gaskets on carb
• adjusted float to specs
• cleaned jets
• did carb 5 times to eliminate any possible issues
• the carb is a mercarb also. 1 mixture screw at bottom. (I did not adjust this or touch it during rebuilds)
• spark cables look in very good condition
• no visible or audible arcing of sparks
• distributor appears solid, clean and in good condition (can't feel any play)
• distributor is electronic - no points or fly weights, just rotor and module
• coil plugs to distributor at center post.

Here is what I did not do:

• did not test for air leak at carb since gaskets are new and there is no fuel leakage around gaskets. (I could do this with spraying wd-40?)
• did not check for intermittent spark (I could do this with timing light on each wire?)
• did not test fuel pressure as I have only a vacuum pump (hand operated) and compression gauge. (any other hack/ideas on what I could use so I don't have to buy a $100 tool I will never need again? I have an a/c gauge, could I somehow use that?)
• did not test giving it extra fuel into carb when in water as I was testing alone at the time.
• did not record high speed advance since no tach for rpm.
• plugs are not changed but look to be in good condition other than the soot.

Am I missing something obvious? I have read every advice column, forum etc on this motor and nothing else works so far.

Please help? I know it's easy to say "take it to a mechanic" or "throw more money at it" but that is not really an option if I can help it because funds are low and non-helpful, self-serving comments like that are of no use to anyone, so please save those for you private "in your head" comments.

Any positive advice would be very helpful please. I live in BC Canada nowhere near a big center that has loads of resources.

 

[Bondo]

Ayuh,...... Welcome Aboard,.......

In general, I believe ya got a fuel delivery issue, could be anything from the tank to the carb,.....

I'd probably start at the carb, but Testin' the entire fuel system is in order,......

1 way to isolate the motor is to run it on a remote tank, with known good clean fresh gasoline,.....

 

[Sashap]

but plugs show that it is getting too much fuel. also, at one point a carb gasket was torn and leaking fuel from the carb to the throttle body so in fact it had extra fuel.
One guy said it seems that timing is off. Where do I get the timing specs for my serial number? I see numbers all over the map online but have no way of knowing if it is for my engine.

 

[Scott Danforth]

start with the carb. any carb sitting for more than 1 year let alone 7 needs to be rebuilt

also, look at the tank, anti-siphon valve, plumbing, etc. any gas older than 1 year, let alone 7 was no longer gas, it was sticky varnish sludge

when you cleaned the carb, you missed something.

 

[TurtleTamer]

As stated, anti-siphon valve.

Is your timing in base mode when it's at zero? If not, your timing is significantly retarded. EST ignition will advance 12-13 degrees once out of base timing.

 

[Sashap]

first, the carb was varnishy smelling for sure. It is perfect now. Second, the fuel in the tank, oddly enough actually ran fine in both my lawn tractor as well as in my truck (thought I would try) but I did flush it all and used new fuel and Seafoam as per my post. Third, please read my post! the carb was rebuilt 5 (FIVE) times just in case. Fourth, please read my post! The entire fuel line system was cleaned, flushed and checked. Nothing was ever plugged, varnished or in bad shape all the way down to the fuel pickup screen in the tank. Fifth, the only thing I never touched was the idle mixture screw and all components/holes etc were cleaned with air and fuel pumped through manually (again, read the post).
When the engine is at idle with the white jumper wires connected at distributor and shift lock disabled, the timing is at 0 Degrees. What is the timing for this engine serial D706324? I can't find it anywhere online.

 

[TurtleTamer]

Shift lock disabled means nothing. With EST, you need +12VDC at the interrupter pin on the distributor. I can't speak for your serial series, but on mine (OMxxxx) the interrupt circuit can be disconnected and jumpered at the switch, near the carb, to provide that 12V. Simply disconnecting the interrupt does nothing. Verify you are at base timing before setting to zero (or spec).

I tried to search your serial for timing info (as I'm sure you have) but found nothing. I don't know what the more experienced guys use as far as a search engine.

I read your post. I feel like you probably have the carb ruled out. Didn't see where you checked the anti-siphon valve. And using a separate can of fresh gas can rule out lines.

 

[Sashap]

I joined the white jumper wires on distributor to put it into base mode. I have found some info saying timing should be 1BTDC for EST ignition. Is that correct? If so, it's only 1 degree off. would that make a difference?
There is a pdf that lists all ignition types for mercruiser. Can't find the link right now as I have printed the pdf. I will check the anti siphon valve again but the tank is all fresh fuel with seafoam. This is crazy. I can't believe after checking everything that it still has issues.

 

[Ed Harris]

I would say its a timing issue. I bought a 3L 1985 model and it backfired through the carb and shut off at about 1200RPMs. Found the timing to be advanced 40 degrees. Idled and cranked fine. Moved it back to 6 BTDC ran like a champ

 

[TurtleTamer]

Jumping the two white wires is only half the procedure. You also have to provide the +12VDC to the interrupt wire. There's a tool for this, which you just unplug the white wired plug from the distributor, hook the tool-plug in and use the supplied alligator clip to send +12v to the interrupt pin, but you don't need that if your original wiring harness allows you to unhook the two black wires at the interrupt switch and connect the purple and white wires. I don't have the experience or resources necessary to see if you do though.

In this video, they have the tool: https://www.youtube.com/watch?v=-swDhVcEdh0

You'll know it goes into base mode because it'll start running very poorly, or if you are at 0BTDC already, it may even quit.

 

[Sashap]

ok, I see that now. I will do that and get back to you. can you confirm that timing is 1 BTDC in this mode?
On top of the engine at the interrupt switch there are two wires coming from it with 1 male end and 1 female end. If I unplug the wires the go from the interrupt and then plug the two wires together rather than to the regular leads, will this do what you described?

 

[TurtleTamer]

That is how mine is set up. It's a 2003 with an OMxxxx serial number. Can't imagine yours would be different, since you describe pretty much the exact same thing I have. With a multimeter, you can confirm +12VDC at the purple wire and check continuity of the white (plus a colored stripe IIRC) to the terminal in your distributor plug. For my engine, it's 1* ATDC for base timing. This puts it at roughly 12* BTDC once taken out of base mode timing at idle. I don't want to tell you wrong for your engine, but that's correct for mine.

 

[fishrdan]

Yes, you need to disconnect the shift interrupt switch wires from the switch, then join them together.

Check the timing now, with the EST white wires disconnected, it should be 10-12* advanced (timing advance is handled by the EST module). If it's sitting at 0-1* the timing is severely retarded.

Use the procedure TT mentioned, EST white wires connected - then joint the shift interrupt wires, this will put the EST into base timing mode (engine stumbles and runs like carp). Adjust timing to what's listed on the engine as there were several settings over the years: 2* BTDC, 0*, 1* ATDC, etc., due to different engine tuning.

 

[Sashap]

I went to do that and now I get no spark at all...
Something happened. Yesterday I removed the tach as is was dead and all corroded. I ordered a new one. This wouldn't cause no spark would it?
So I started to test the coil and where it says I should get .4 ohms I get .7 and other areas are returning weird numbers. So then I went to test pickup coil on alternator and it shows even weirder numbers. Could it have been going bad and this just was the end? I am going to order a new coil, pickup coil, distributor cover and rotor while I'm at it.
If it's not one thing, it's your mother.

I will keep you posted when I get the parts on Tuesday.
Cross your eyes that everything turns out ok!
Thanks all for the help so far.

 

[TurtleTamer]

Coil is probably fine. They usually aren't the issue, and anything under an ohm should be fine. It's literally just wire.

A bad tach can ground out causing a no spark, so having it removed, as long as your wires are isolated, should be fine.

Make sure to *disconnect both black wires from the interrupter switch and *connect the purple and white/grn-stripe wires to provide +12V to the dizzy. Doing this out of order can cause a no spark. Don't ask me how it's done wrong. I just know it can be done wrong as I've killed my engine going in and out of base mode. Don't panic. Ensure the white ones are jumped properly as the connections can corrode.

 

[fishrdan]

Did you have the engine running before attempting to put the EST into base timing mode?

If you jumpered the shift interrupt switch wires with the engine off, it probably (???) won't start. I've never tried it, but that's the purpose of the shift interrupt switch, to kill the ignition (momentarily while shifting out of gear).

1- start and warm engine
2- jumper white wires on the distriburtor
3- connect shift interrupt switch wires (now the engine should be running poorly - because the EST is in base timing mode)

 

[Sashap]

the tach wire harness in the dash is a 3 wire set that is covered in rubber so I'm sure that's ok and not an issue to ground out.
Could the icm be bad? no way to test it that I can find. I did see that the identical one from GM (even this one is stamped GM) is only $65 vs the $115+ for "marine".
I am going to replace the cap and rotor anyway as it is cheap. Also the pickup coil is quite cheap and the wires in this one are kind of iffy anyway.
I tried starting with the wires all connected in normal run mode and was then going to do the timing connection with a warm engine (off) like I did before. It's just weird that there is no spark at all from anything. The safety lanyard was plugged in beside the key and I did not touch anything else other than the dash. I did notice a wire came loose from the hour meter but I figure this is just the power to the key that makes the meter start ticking when the key is on.
The only thing I changed from last time I had it running was that I sprayed some wd-40 on the pickup coil to clean it out as it was covered in a rust like powder. Checking the resistance between the white/green wires on the pickup coil I get a steady 813 ohms however when I move the coil around the number jumps around and the component test states that this could be a cause of some intermittent issues. Again, it's a cheap part and it's going to rain for a day or two anyway so I can wait.
Thanks again, keep you posted.

 

[Sashap]

Did you have the engine running before attempting to put the EST into base timing mode?

If you jumpered the shift interrupt switch wires with the engine off, it probably (???) won't start. I've never tried it, but that's the purpose of the shift interrupt switch, to kill the ignition (momentarily while shifting out of gear).

1- start and warm engine
2- jumper white wires on the distriburtor
3- connect shift interrupt switch wires (now the engine should be running poorly - because the EST is in base timing mode)

The last time it was running when I check the timing and confirmed it was at 0 Degrees (3 days ago) I started it, warmed it and then stopped it to do the jumper connection and the shift interrupt swap/bypass. Then I started it in that mode.

But, now I have this frickin no spark issue to deal with...

 

[Sashap]

I would say its a timing issue. I bought a 3L 1985 model and it backfired through the carb and shut off at about 1200RPMs. Found the timing to be advanced 40 degrees. Idled and cranked fine. Moved it back to 6 BTDC ran like a champ

I just need to confirm the REAL timing number. The pdf I found states that for EST ignition it is 1BTDC. I am (or at least was before this no spark issue) at 0 Degrees. That is soooo close. Does is matter for 1 degree here or there?

 

[Sashap]

That is how mine is set up. It's a 2003 with an OMxxxx serial number. Can't imagine yours would be different, since you describe pretty much the exact same thing I have. With a multimeter, you can confirm +12VDC at the purple wire and check continuity of the white (plus a colored stripe IIRC) to the terminal in your distributor plug. For my engine, it's 1* ATDC for base timing. This puts it at roughly 12* BTDC once taken out of base mode timing at idle. I don't want to tell you wrong for your engine, but that's correct for mine.

I will check the purple wire voltage etc. I get 0degrees in idle regardless of the base mode as far as I can tell. Although I can't confirm until I get the spark back. I can't check until tomorrow afternoon (I'm beat right now and just don't have it in me).
Thanks.